I've seen this story around the net today, so I thought I would link to the short research paper by Felisa Smith and colleagues: "Methane emissions from extinct megafauna." The idea is that the extinction of the mammoths, mastodons, horses, camels, steppe bison and other big herbivores may have helped trigger the Younger Dryas:
Our calculations suggest that the late Pleistocene extinction resulted in a decreased annual methane flux of ~9.6 teragrams (Tg) (range 2.3 to 25.5 Tg; Supplementary Tables T1 and T2). Interestingly, ice-core records of methane concentration reveal an abrupt drop of >180 ppbv at the onset of the Younger Dryas cold event, about 12,800 years ago (Fig. 1). The drop seems to be synchronous with the extinction of New World megafauna.
The reduction in atmospheric methane is documented by ice cores, and the paper shows that the loss of megafauna ight be enough to explain all of the reduction, or at least a large fraction of it. Methane is a more powerful greenhouse gas than carbon dioxide or water vapor. How much difference would it have made? That's the unanswered question.
In light of my "Anthropocene" comments, I have to mention the way they end their letter:
Thus, we propose that the onset of the 'Anthropocene' should be recalibrated to 13,400 years before present, coincident with the first large-scale migrations of humans into the Americas.
Holy unnecessary interval, Batman! If we're going to make a geological epoch begin at the onset of the Younger Dryas, shouldn't it obviously be the Holocene?
UPDATE (2010-05-25): From a reader:
Given this new hypothesized cause for the onset of the Younger Dryas ends up being validated, a more precise/appropriate name should then be "Flatulocene".
Yes, well, everyone, just tell them you caught wind if it here.
Smith FA, Elliott SM, Lyons SK. 2010. Methane emissions from extinct megafauna. Nature Geosci (in press) doi:10.1038/ngeo877